Coal gasification units offer a viable alternative to flue gas scrubbing for the utilization of high sulfur coals in a commercial utility steam generator. Coal gasifiers, in conjunction with combined cycle power generation, offer a significant decrease in the plant heat rate, resulting in cost savings in the production of electricity. One of the most attractive coal gasifier designs is an entrained, upward gas flow unit firing pulverized coal to produce a low BTU and medium BTU product gas.
A benefit of the reaction of pulverized coal and air, substoichiometrically, at high temperatures (2500 F.-3500 F.) is that all ash contained in the coal is melted, that is liquified, so as to be removable by flowing under the influence of gravity to some type of ash-handling system. The melted ash (slag) gravitates down the walls and the sloped floor of the reactor to some opening, commonly called a slag tap, which provides an outlet for slag flowing from the reactor so it can be deposited in the slag-handling system below the furnace.
Throughout the history of entrained flow reactors with vertical gas outlets, problems have arisen with the lower portions of the reactor being cooler than the upper regions of the reactor due to localized heat transfer, vapors rising from the quench tank, and low stoichiometry due to poor mixing. A result is that slag flow is obstructed due to the temperature effect on slag viscosity and the slag becomes difficult to tap. Buildup of solidifying slag around the tap hole area, with the possibility of a slag hole pluggage, can make long-term operation difficult.
In order to alleviate the buildup condition, three major slagging aids in the present art are: first, the use of fluxing agents to decrease the slag viscosity; second, the use of ignitors at the tap hole region to add supplemental heat to the tap region; and third, the use of hot reverse gas from the combustor through the slag hole to some point downstream of the reactor. Not all coals can be mixed with fluxing agents to decrease the slag viscosity and fluxing agents can result in operational problems downstream of the reactor. The use of oil and gas ignitors at the slag tap to add supplemental heat requires the constant use of oil and natural gas, is not always effective, and is also expensive. Part of the hot gases has been reversed to flow through the slag tap and normally vented downstream of the reactor where they cannot be efficiently utilized. Reverse gas can be done utilizing a recirculating fan which vents the gases back to the reactor, but this requires cooling of the reverse gas from 2500 F. to 600 F. to protect the fan and the use of an inefficient fan. Both result in a large energy penalty.
Some arrangement is needed to keep the slag hot with the back-flow product gas from the reactor without incurring the penalties of using an induced draft fan for this purpose.